By Windiyanto Ajie Nugroho – (Thesis M.Si Sriwijaya University, 2012), Supervised by Prof. Bart Schultz, PhD, MSc; Prof. Ir. Robiyanto H. Susanto, PhD, M.Agr.Sc; F.X. Suryadi, PhD, MSc.
Kudus Regency is one of the regencies in Central Java Province. It is bordered by Jepara and Pati Regency in the North, Pati Regency in the East, Grobogan and Pati Regency in the South, and Demak and Jepara Regency in the West.
Located at the southern part of Mount Muria, Kudus Regency has a total area of 42,500 ha which divided into 9 districts and 132 villages. Its elevation is between 3 to 1,500 m+MSL with average elevation of 55 m+MSL. Since it is located on the mountain slope, most of its area, mainly in the northern part, is hilly. The southern part is relatively flat and prone to flooding, especially in the south-western part which is bordered by Demak Regency. Srep Sub River Basin is named after the river in its most upstream part, but after joins Tunggul River it flows to the downstream area as Gelis River that flows right at the heart of the capital city of Kudus Regency. The flooding that happens in that area is usually the result of excess runoff from the upstream part of Srep Sub River Basin, severed by the high water level from Wulan River where Gelis River flows.
Srep Sub River Basin has an elevation ranging from 6.3 to 1,480 m+MSL with the total area of 7,560 ha. From the northern part to the middle part, Srep Sub River Basin’s topography is hilly with more than 40% slope in certain areas, and becomes milder and relatively flat in the southern part where ultimately bordered with Wulan River. The upstream part of Srep Sub River Basin is used mainly for forestry and agricultural areas, while the downstream part is mainly used as settlement and industrial areas besides the location of the main road that connects the northern parts of Java Island that will be affected greatly when flooding occurs. Therefore, if flooding problems occur in this sub river basin they will affect greatly the life of Kudus Regency inhabitants.
There are three dams built along Srep River that serve for irrigation purpose. From the upstream direction to downstream direction, those dams are: Karanggayam Dam, Kedung Gupit Dam, and Ploso Dam respectively.
The evaluation of the flooding condition in Srep Sub River Basin is conducted by constructing the DUFLOW model that represents the condition of Gelis River. The results of DUFLOW modelling are the water level and the surface level. From the disparity between the water level and river surface level, the flood levels can be determined for certain areas along the river. The results are then brought into GIS to model how deep the floods inundate the area.
DUFLOW modelling and GIS results show that the downstream part of Ploso Dam where the slope is relatively mild flooded up to 0.9 m when 2% chance of occurrence rainfall rate (50 years of retur period) occurs, while the upstream part of Ploso Dam is flood-free because of the topography condition which is relatively steep. The condition of the downstream of Ploso Dam is severed by sedimentation that reduces the capacity of the river to convey water and the high water level of Wulan River where Gelis River flows.
Three scenarios are proposed to overcome the flooding problem. Those are: dredging, retention pond, and dike heightening. The results show that dredging alone is not sufficient to overcome the flooding problem. However, dredging is a necessary maintenance activity for the river so it must be done regularly to maintain the condition of the river.
The second scenario, retention pond addition is sufficient to overcome most of the flooding problems, except for the most downstream part of Gelis River. However, the area needed for a retention pond is quite large. It needs 60 ha of area with 5 m depth which is not an easy option to choose, although it is actually only 1% of the total area of Srep Sub River Basin which is 7,560 ha. 60 ha of non-settlement area are available in the upstream part of Kedung Gupit Dam, where the retention pond location is proposed, but the real implementation is not an easy decision to do. The alternative option for this scenario is to build not only one big retention pond, but several smaller retention ponds that would be located along Gelis River, but still it is not an easy decision to be implemented.
The last scenario is dike heightening. This scenario has two alternatives: dike heightening along Ploso section of Gelis River; and dike heightening only at the most downstream part of Gelis River combined with retention pond addition. The first alternative is the most reasonable scenario. Since widening of the river is impossible, and the addition of a retention pond is not easy to be implemented, dike heightening up to 1 m is proven to overcome the flooding problem in Srep Sub River Basin. However, the second alternative has its own huge advantage. While the dike heightening is the most reasonable scenario, it only solves the flood problem. On the other hand, the alternative of retention pond addition is not only solving the flooding problem, but also solving the problem of water scarcity for irrigation in Srep Sub River Basin during the dry season.
Keywords: DUFLOW modelling, Geographic Information System, flood protection